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  1. #1
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    snapped rear derailleur hanger

    Recently I got some new FSA SLK carbon cranks to replace my Ultegra ones. I noticed that the width is slightly different (still can't quite get the front derailleur perfect with them, it rubs ever so slightly when in the small ring/big cog combo).

    More of a concern was the way the chain would occasionally "slip" when I shifted down to the small ring. Usually I shift two cogs smaller, then shift the front gear when I do that. Literally the chain wouldn't engage and the cranks just spun uselessly, as if it fell between two cogs at the back. A few pedal strokes and a quick upshift of the rear derailleur and it was fine.

    Then today, I'm riding up a hill, nothing too crazy, decide to shift to the small ring -- not under heavy load, I always consciously let up slightly when shifting between rings -- and the rear derailleur hanger snaps completely off sending the whole mess into my spokes. I stopped and disentangled it, the chain is fine, the derailleur is fine, the hanger is sheared in two.

    Doing some searching, it sounds like chain suck is the culprit? But the rings are new, the chain is new, cassette is in good shape. This problem has only started since I got the FSA crankset with its narrower width.

    Any idea what happened, why, and how to prevent it in the future?

  2. #2
    Old fart JohnDThompson's Avatar
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    I suspect you're right that it is "chain suck." When you shift to the smaller ring the chain briefly goes quite slack, increasing the chance it can be sucked into the wheel. The small difference in offset between the old and new cranks can be enough to trigger this. The quickest and cheapest way to deal with this is to install an anti-suck device. If that doesn't fix it you may need to adjust the chainline using spacers or whatever your bottom bracket allows.

  3. #3
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    John and I usually agree, but in this case we don't. Your description sounds nothing like chain suck, which refers to a chain failing to dis-engage off the chainring's bottom and getting carried around the back until it jams under the chainstay.

    Your initial problem of the chain getting thrown off the inside sounds more like a simple FD trim adjustment, and you probably need to fine tune the FD limits, and possibly other adjustments. It could also be the result of shifting with excess tension which makes the chain spring inward when the shift gate comes by.

    The rear hanger is probably not directly related, but possibly when the chain falls off the front, the idler cage spins bringing the upper pulley into contact with the larger sprockets. It could also be a totally unrelated problem of a mis-adjusted inner limit screw allowing the RD to get snagged by the spokes.

    At this point you need a new hanger, and need to inspect both the chain for damaged links and the RD itself before riding. If you're up to it do a thorough adjustment on both derailleurs so shifting is 100% before riding, or let a pro go over it and make sure all is OK. Hanger, RD and chain damage is expensive and 99% avoidable, though even if all is perfect bad luck like a stick getting jammed where it doesn't belong still happens. That's why they make breakaway hangers.
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  4. #4
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    Thanks for the replies, both.

    I do all my own maintenance and repairs so feel pretty confident replacing the mech hanger (ironically I crashed about a month ago and bent it so this hanger is quite new! but I was having the weird chain slip problem long before I crashed). Rear mech is never an issue for getting it perfect on any of my bikes, but I absolutely hate trying to adjust the front mech to get it right. I've got the inner screw set as far as it can go inwards and still the rubbing on the small ring/big cog. So annoying. Maybe I need to install a barrel adjuster somewhere on the front derailleur cable.

    The chain slip was actually on the back, not the front. It was almost as if it was slipping off the jockey wheels if that makes any sense. A friend of mine who had a similar problem with his bike. He said
    "What I think it is, as I witnessed it doing it on the bike stand or starting to) is the chain jumps off one of the jockey wheels in the rear derailleur. The chain then gets stuck in the cage and the whole mech is pulled in the direction of the chain. There is so much force due to your forward momentum that it snaps the mech (twisting) and the hanger as that is the weakest point."
    Spacers sounds like the solution. Fwiw I ride a 48cm Cervelo S1 with compact cranks, so the chainstays are pretty short (the whole bike is quite tiny) and I guess the tolerances are pretty low if a few mm offset in cranks can cause a problem like this.

  5. #5
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    Before re-spacing your cranks see if it's indicated. If your FD cannot move inboard enough (back off the limit and remove the cable) then moving the crank out is warranted, otherwise it's just a question of careful FD set up.

    I hate crapping with the pinchbolt trial and error like an idiot, so consider an in-line or lever adjuster de-riguer on index triple fronts. Doubles and non-index fronts don't need it because trim is really set by the limit screws and/or trimming non-index levers.



    As to the skipping in the back, it's possible that the RD is set too close to the cassette, causing the jockey wheel to engage the sprocket through the chain. That can cause skipping, and or the RD to get snagged and pushed back. As a guide there should be at least 1" of chain between the tangent of the pulley and sprocket in all combinations, so the pulley is leading the chain onto the sprocket, but well forward (along the chain) at all times.

    It's also possible that you have some stiffness in the chain. Back pedal slowly holding the idler cage so there's slack in the lower loop and watch for links that don't straighten immediately under the weight of the chain alone. Stiff links call for a wash, dry and styling (oops) re-lube.
    FB
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    An ounce of diagnosis is worth a pound of cure.

    “Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin

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    WARNING, I'm from New York. Thin skinned people should maintain safe distance.

  6. #6
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    Quote Originally Posted by FBinNY View Post
    Before re-spacing your cranks see if it's indicated. If your FD cannot move inboard enough (back off the limit and remove the cable) then moving the crank out is warranted, otherwise it's just a question of careful FD set up.
    I've got the FD moved inboard as far as it will go, I've been fiddling with the angle of the cage to make it not rub when on small ring/big cog, though it's been a faff for sure. Like I said, it worked great with the Ultegra cranks, so it's the spacing of the FSA cranks that's the problem. I only went to them because I wanted 165mm cranks... now wondering if it's worth the hassle! I do like that they are spinnier though.

    Quote Originally Posted by FBinNY View Post
    As to the skipping in the back, it's possible that the RD is set too close to the cassette, causing the jockey wheel to engage the sprocket through the chain. That can cause skipping, and or the RD to get snagged and pushed back. As a guide there should be at least 1" of chain between the tangent of the pulley and sprocket in all combinations, so the pulley is leading the chain onto the sprocket, but well forward (along the chain) at all times.

    It's also possible that you have some stiffness in the chain. Back pedal slowly holding the idler cage so there's slack in the lower loop and watch for links that don't straighten immediately under the weight of the chain alone. Stiff links call for a wash, dry and styling (oops) re-lube.
    Replaced the hanger, had to replace the chain too (some of the links were actually twisted from the force of the hanger ripping apart), readjusted the B-screw to put the jockey wheels further away from the cogs, and we'll see how it goes. It looks like the cage may be a slight bit twisted as well so I may have a go at straightening it if I still have problems shifting.

    Thanks again for the help.

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